Suzaku X-Ray Observation of the Dwarf Nova Z Camelopardalis at the Onset of an Optical Outburst

We present the result of a Suzaku X-ray spectroscopic observation of the dwarf nova Z Camelopardalis, which was conducted by chance at the onset of an optical outburst. We used the X-ray Imaging Spectrometer (a 38 ks exposure) and the Hard X-ray Detector (34 ks) to obtain a 0.35-40 keV spectrum simultaneously. Spectral characteristics suggest that the source was in the X-ray quiescent state despite being in the rising phase of an outburst in the optical band. The spectrum shows a clear signature of circumstellar absorption in excess of interstellar absorption and the reprocessed emission features of Fe fluorescence and Compton scattering. The extra absorption is explained due to partial coverage by either neutral or ionized matter. We found a spectral change during the observation, which is attributable only to the change in the circumstellar absorption. Such an X-ray spectral variation is reported for the first time in dwarf novae. We speculate that the variation in the circumstellar absorption is interpreted as a time-varying disk wind or geometrically flaring disk around the white dwarf during the propagation of a heat wave inward along the accretion disk at the beginning of the outburst, in which optical outburst and X-ray quiescent states co-exist.

Cross-calibration of the X-ray Instruments onboard the Chandra, INTEGRAL, RXTE, Suzaku, Swift, and XMM-Newton Observatories using G21.5-0.9

Context. The Crab nebula has been used as a celestial calibration source of the X-ray flux and spectral shape for many years by X-ray astronomy missions. However, the object is often too bright for current and future missions equipped with instruments with improved sensitivity. Aims. We use G21.5-0.9 as a viable, fainter substitute to the Crab, which is another pulsar-wind nebula with a time-constant powerlaw spectrum with a flux of a few milli Crab in the X-ray band. Using this source, we conduct a cross-calibration study of the instruments onboard currently active observatories: Chandra ACIS, Suzaku XIS, Swift XRT, XMM-Newton EPIC (MOS and pn) for the soft-band, and INTEGRAL IBIS-ISGRI, RXTE PCA, and Suzaku HXD-PIN for the hard band. Methods. We extract spectra from all the instruments and fit them under the same astrophysical assumptions. We compare the spectral parameters of the G21.5-0.9 model: power-law photon index, H-equivalent column density of the interstellar photoelectric absorption, flux in the soft (2-8 keV) or hard (15-50 keV) energy band. Results. We identify the systematic differences in the best-fit parameter values unattributable to the statistical scatter of the data alone. We interpret these differences as due to residual cross-calibration problems. The differences can be as large as 20% and 9% for the soft-band flux and power-law index, respectively, and 46% for the hard-band flux. The results are plotted and tabulated as a useful reference for future calibration and scientific studies using multiple missions.

X-Ray Spectroscopy of the Classical Nova V458 Vulpeculae with Suzaku

We conducted a target of opportunity X-ray observation of the classical nova V458 Vulpeculae 88 days after the explosion using the Suzaku satellite. With a 20 ks exposure, the X-ray Imaging Spectrometer detected X-ray emission significantly harder than typical super-soft source emission. The X-ray spectrum shows K lines from N, Ne, Mg, Si, and S, and L-series emission from Fe in highly ionized states. The spectrum can be described by a single temperature (0.64 keV) thin thermal plasma model in collisional equilibrium with a hydrogen-equivalent extinction column density of ~3e21/cm2, a flux of ~1e-12 erg/s/cm2, and a luminosity of ~6e34 erg/s in the 0.3-3.0 keV band at an assumed distance of 13 kpc. We found a hint of an enhancement of N and deficiencies of O and Fe relative to other metals. The observed X-ray properties can be interpreted as the emission arising from shocks of ejecta from an ONe-type nova.

X-Ray Observations of the Sagittarius D HII Region toward the Galactic Center with Suzaku

We present a Suzaku X-ray study of the Sagittarius D (Sgr D) HII region in the Galactic center region. Two 18x18 images by the X-ray Imaging Spectrometer (XIS) encompass the entire Sgr D complex. Thanks to the low background, XIS discovered two diffuse sources with low surface brightness and obtained their high signal-to-noise ratio spectra. One is associated with the core of the Sgr D HII region, arising from the young stellar cluster. The other is a new object in the vicinity of the region. We also present 3.5 cm and 6.0 cm radio continuum maps of the new source using the 100 m Green Bank Telescope. We conclude that the source is a new supernova remnant (SNR; G1.2--0.0) based on: (1) the 0.9+/-0.2 keV thermal X-ray spectrum with emission lines from highly ionized atoms; (2) the diffuse nature with an apparent extent of ~10 pc at the Galactic center distance inferred from the X-ray absorption (~8.5x10^{22} cm^{-2}); and (3) the nonthermal radio continuum spectral index (~-0.5). Our discovery of an SNR in the Sgr D HII region leads to a revision of the view of this system, which had been considered to be a thermal HII region and its environment.

Suzaku X-Ray Spectroscopy of a Peculiar Hot Star in the Galactic Center Region

We present the results of a Suzaku study of a bright point-like source in the 6.7 keV intensity map of the Galactic center region. We detected an intense FeXXV 6.7 keV line with an equivalent width of ~1 keV as well as emission lines of highly ionized Ar and Ca from a spectrum obtained by the X-ray Imaging Spectrometer. The overall spectrum is described very well by a heavily absorbed (~2x10^{23}cm^{-2}) thin thermal plasma model with a temperature of 3.8+/-0.6 keV and a luminosity of ~3x10^{34} erg s^{-1} (2.0--8.0 keV) at 8 kpc. The absorption, temperature, luminosity, and the 6.7 keV line intensity were confirmed with the archived XMM-Newton data. The source has a very red (J-Ks=8.2 mag) infrared spectral energy distribution (SED), which was fitted by a blackbody emission of ~1000 K attenuated by a visual extinction of ~31 mag. The high plasma temperature and the large X-ray luminosity are consistent with a wind-wind colliding Wolf-Rayet binary. The similarity of the SED to those of the eponymous Quintuplet cluster members suggests that the source is a WC-type source.

Suzaku Detection of an Intense X-Ray Flare from an A-type Star HD161084

We report a serendipitous detection of an intense X-ray flare from the Tycho reference source HD 161084 during a Suzaku observation of the Galactic Center region for 20 ks. The X-ray Imaging Spectrometer (XIS) recorded a flare from this A1-type dwarf or subgiant star with a flux of 1.4x10^{-12} erg s^{-1} cm^{-2} (0.5--10 keV) and a decay time scale of 0.5 hr. The spectrum is hard with a prominent Fe XXV K alpha emission line at 6.7 keV, which is explained by a 5 keV thin-thermal plasma model attenuated by a 1.4x10^{21} cm^{-2} extinction. The low extinction, which is consistent with the optical reddening, indicates that the source is a foreground star toward the Galactic Center region. Based on the spectroscopic parallax distance of 530 pc, the peak X-ray luminosity amounts to 1x10^{32} erg s^{-1} (0.5--10 keV). This is much larger than the X-ray luminosity of ordinary late-type main-sequence stars, and the X-ray emission is unattributable to a hidden late-type companion that comprises a wide binary system with the A-star. We discuss possible natures of HD 161084 and suggest that it is most likely an interacting binary with elevated magnetic activity in the companion such as the Algol-type system. The flux detected by Suzaku during the burst is 100 times larger than the quiescent level measured using the archived XMM-Newton and Chandra data. The large flux amplification makes this star a unique example among sources of this class.